A lightly loaded electric motor will operate with a larger current lag than a fully loaded electric motor. The motor power factor is the cosine of the current lag, when the current lag is expressed in electrical degrees. The lightly loaded motor will operate more efficiently if the current flow to the motor is controlled in such a way as to decrease the current lag. A continuing problem in the design and operation of motor controllers is to select the optimum current lag for operation of a lightly loaded motor.
A method suggested by Fitzpatrick, et al., U.S. Pat. No. 4,581,568, issued Apr. 8, 1986 for selecting an optimum current lag, is to determine the minimum value of the current lag measured as a lightly loaded motor starts and approaches a full speed condition. The optimum value is then computed by adding a predetermined amount to the measured minimum value of the current lag. The addition of a predetermined amount helps to avoid choosing a reference value of current lag that is too small. Although the selection of a reference current lag, as disclosed by Fitzpatrick, et al., approximates a current lag for a fully loaded motor, the problem of automatically selecting a better approximation that does not depend on the addition of an arbitrary predetermined amount remains unsolved.